A Light Emitting Diode (LED) is a solid state device that converts electrical energy to light. Light is emitted from an active layer of semiconductor materials sandwiched between oppositely doped layers when a voltage is applied across the doped layers. There are many different LED device structures that are made of different materials and have different structures and perform in different ways. Some emit laser light, and others generate non-monochromatic and non-coherent light. Some are optimized for performance in particular applications. Some are high power devices and others are not. Some emit light as infrared radiation, whereas others emit visible light of various colors, and still others emit ultraviolet light. Some are expensive to manufacture, whereas others are less expensive. For commercial general lighting applications, a blue LED structure is often used. Such a blue LED having a Multiple Quantum Well (MQW) active layer involving Indium-gallium-nitride may, for example, emit non-monochromatic and non-coherent light having a wavelength in a range from 440 nanometers to 490 nanometers. A phosphor coating is then typically provided that absorbs some of the emitted blue light. The phosphor in turn fluoresces to emit light of other wavelengths so that the light the overall LED device emits has a wider range of wavelengths. The overall LED device that emits the wider range of wavelengths is often referred to as a “white” LED.
Although gallium-nitride substrate wafers are available, they are very expensive. The epitaxial layers of commercial blue LEDs are therefore typically grown on wafers of other types of substrates such as, for example, sapphire wafers. These other substrates are, however, still undesirably expensive. Common integrated circuits of the type employed in personal computers are generally fabricated on silicon substrates. As a result of the high volumes of silicon substrates produced for the computer industry, silicon substrates are relatively inexpensive as compared to sapphire substrates. Moreover, second hand semiconductor processing equipment for processing silicon substrate wafers is often available at low prices due to the fact that integrated circuit fabrication companies frequently upgrade their fabrication facilities in order to keep up with advances in integrated circuit manufacturing technology. It therefore would be desirable from a cost point of view to be able to fabricate GaN-based LEDs on relatively inexpensive silicon substrate wafers and to use the available second hand semiconductor processing equipment for processing such silicon wafers, but there are many problems with growing high quality GaN epitaxial layers on silicon substrates.
Many of the problems associated with growing high quality GaN epitaxial layers on silicon substrates derive from the fact that the lattice constant of silicon is substantially different from the lattice constant of GaN. When GaN is epitaxially grown on a silicon substrate, the epitaxial material being grown may exhibit an undesirably high density of lattice defects. If the GaN layer is grown to be thick enough, then stress within the GaN layer may result in a type of cracking in the latter grown portions of the GaN material. Moreover, silicon and GaN have different coefficients of thermal expansion. If the temperature of a structure involving GaN disposed on a silicon substrate is increased, for example, then the silicon material portion of the structure will expand at a different rate from the rate at which the GaN material expands. These different rates of thermal expansion give rise to stress between the various layers of the LED device. This stress may cause cracking and other problems. Furthermore, it is difficult to grow GaN on silicon substrate because GaN is a compound material and Si is an elemental material. The transition from nonpolar to polar structure, combined with the substantial lattice mismatch, generates defects. For these and other reasons, the epitaxial LED structure portions of most commercially-available white LED devices are not grown on silicon substrates. Improved processes and structures for fabricating blue LEDs on silicon substrates are sought.
The manufacture of blue LEDs grown on silicon substrates also typically involves wafer bonding. In one prior art process, an epitaxial blue LED structure is grown on a non-GaN substrate to form a device wafer structure. A layer of silver is formed on the epitaxial LED structure to function as a mirror. A barrier metal layer involving multiple periods of platinum and titanium-tungsten is then disposed on the silver mirror. The platinum layer in each period is a thin 60 nm layer. The titanium/tungsten layer in each period is about 10 nm thick and involves about approximately ninety percent tungsten. Five or more such periods are provided. Once the device wafer structure has been formed in this way, a carrier wafer structure is wafer bonded to the device wafer structure. The original non-GaN substrate of the device wafer structure is then removed and the resulting wafer bonded structure is singulated to form LED devices. In this prior art process, a layer of bonding metal is used to wafer bond the carrier wafer structure to the device wafer structure. This bonding metal layer involves a gold/tin sublayer. When the gold/tin sublayer is melted during wafer bonding, tin from this gold/tin sublayer does not penetrate into the silver layer due to the thickness of the multi-period barrier metal layer and due to a short high temperature cycle being used to melt the bonding metal. This prior art process is recognized to work well.